1. Field of the Invention
The present invention relates generally to horizontal band saw machines in which a saw head assembly carrying a band saw blade is lowered during a cutting operation in order to feed the band saw blade into an underlying material to be cut, and, more particularly, pertains to methods and apparatus for controlling the feeding of the band saw blade in horizontal band saw machines.
2. Description of the Prior Art
Horizontal band saw machines conventially comprise a saw head assembly in which a flexible endless band saw blade is trained around a pair of wheels so that it may perform cutting operations when the wheels are driven. The saw head assembly is so arranged as to be raised and lowered in relation to an underlying base supporting the material to be cut. This movement can occur around a hinge pin or along a single or plural guide posts in response to operation of a driving means such as a hydraulic motor. Thus, in cutting operations, the saw head assembly is lowered from the raised position toward the base so as to feed the band saw blade into the material to be cut which has been placed on the base. Also, in most prevailing horizontal band saw machines, the saw head assembly is so arranged as to be raised by a hydraulic motor of a cylinder type when the hydraulic fluid is supplied thereinto and lowered by its own gravity when the hydraulic fluid is drained from the hydraulic motor. In this connection, it is desired that the upper limit of the movement of the band saw blade be made a certain distance higher than the top of the material to be cut so as to prevent the band saw blade from being damaged by the material being fed, since most of materials are usually uneven.
In such horizontal band saw machines, it is desired to feed the band saw blade into the material to be cut so that the cutting action may be performed always at a constant optimum cutting rate which is defined as area (square millimeters) of cut per unit of time. Since many materials such as round bars and shape steels vary in cross-sectional length as they are cut, it is necessary to feed the band saw blade in correlation with the cross-sectional length of the material being cut so as to maintain an optimum cutting rate. Of course, if the band saw blade is not fed at an optimum cutting rate according to the cross-sectional length of the material being cut, it will work too hard and wear prematurely or otherwise work inefficiently.
For the purpose of performing cutting operations at a stated cutting rate according to the cross-sectional length of the material being cut, the band saw blade is conventionally arranged to be fed with a uniform load or feeding force under the so-called load control or pressure control. In order to maintain the load or feeding force constant under the load control, the pressure of the hydraulic motor for the saw head assembly is controlled while the saw head assembly is lowered to feed the band saw blade into the material to be cut. The load control is intended to maintain the cutting resistance constant so as to feed the band saw blade at a uniform cutting rate, since the cutting rate is generally proportional to the cutting resistance. Thus, under the conventional load control, the feeding speed of the band saw blade is controlled as a function of the cross-sectional cut length of the material to be cut so as to keep the cutting resistance constant, while the driven speed of the bandsaw blade is kept unchanged.
The conventional load control, however, is defective especially in that it is based on the premise that the cutting resistance per unit of feeding amount or feeding speed of the bandsaw blade is always uniform or constant. In fact, the cutting resistance per unit of feeding amount of the band saw blade has a tendency to increase as the feeding amount decreases. Accordingly, when the feeding amount is decreased in order to maintain the cutting resistance constant in response to an increase in the cross-sectional length of the material being cut, for instance, the cutting resistance per unit of feeding amount will have a tendency to further increase. The cutting resistance per unit of feeding amount will markedly increase, especially when cutting difficult-to-cut materials such as stainless steels which have to be cut at a low feeding speed or small feeding amount. Thus, the difficult-to-cut materials cannot be cut at a uniform optimum cutting rate by the conventional load control in the horizontal machine, although the easy-to-cut materials which can be cut at a higher feeding speed or larger feeding amount can be cut at a substantially uniform cutting rate under the same conventional load control.
Furthermore, conventional load control has been defective in that it is based on the premise that the cutting resistance will increase or decrease only as a function of the changes of the cross-sectional length of the material being cut. However, the fact is that the cutting resistance will increase regardless of the cross-sectional length of the material being cut as the band saw blade becomes worn. Thus, when the wear of the band saw blade is increased during cutting operations under the conventional load control, the feeding speed of the band saw blade will be dropped without increase of the cross-sectional length of the material being cut with the result that the cutting rate will drop.